Control of a small mass flow rate of gas in a typical range from 1 standard cubic centimeter per minute (sccm) down to 0.1 sccm is challenging. However this is often required in a variety of industrial and analytical applications, such as semiconductor manufacturing and gas chromatography. A controller typically comprises a sensing device and a control device. The challenge comes from both sensing side and control side. The present invention addresses problems on sensing side.
FIG. 1 shows a schematic of a conventional mass flow controller (MFC) 100. In its simplest form, an MFC 100 consists of a proportional valve (PV) 101, a mass flow sensor (FS) 102, and control electronics (i.e., a control device) 103 to create a feedback loop. The mass flow sensor 102 measures the flow rates and sends the electric signals to the control device 103, which determines the flow rates based on the signals received. The determination often is based on comparison with a standard curve stored in the control device. Based on such determinations, the control device 103 then regulates the proportional valve 101 to produce the desired flow rates.
In FIG. 1, the mass flow sensor 102 is shown to be downstream of the proportional valve 101. In some designs, the mass flow sensor 102 may be upstream of the proportional valve 101. Furthermore, there are other MFCs that have the mass flow sensors in the form of a bypassing tube that passes a fixed fraction of what flows via a main passageway. The bypass tube design is typically used when it is desirable to expand the measurement ranges.
As noted above, a mass flow sensor (such as 102 shown in FIG. 1) produces flow dependent electric signals, which are used by the control electronics to determine appropriate regulation of the proportional valve (such as 101 shown in FIG. 1) in order to control the gas flow. The accuracy of the electric signals produced by the mass flow sensor is critical for the accurate control of the flows. Therefore, calibration is needed to establish a relation between such signals and actual mass flow rates, which are typically expressed in standard cubic centimeter per minute (sccm) or other equivalent units.
Calibration of a mass flow sensor starts with determining a sensor signal at zero-flow conditions (namely, the “zero-offset” value). Then, signals for several pre-defined non-zero flow rates are measured, and the zero-offset value is subtracted from the values of these signals to construct a calibration curve. Once such a calibration curve is created, it is usually stored in a non-volatile memory in the MFC and constantly referenced by the feedback loop control.
The above described setup and calibration generally work well when the flow rates are not too small. For small mass flow rates, it is difficult to measure the signals accurately by the mass flow sensors because these signals are close to the zero-offset, and it is difficult to keep a stable zero-offset regardless of the mass flow sensor types.
Zero-offset could be sensitive to many variables, such as temperatures, pressures, sensor mount orientations, etc, depending on the MFC designs. Most common and significant is the temperature sensitivity, followed by the pressure sensitivity, of the sensors. The sensors may also exhibit long-term drifting due to a variety of reasons, e.g., sensor internal stress relief. Consequently, zero-offset is commonly characterized by its temperature coefficient, pressure coefficient, and time coefficient, respectively, for commercial products.
Currently, temperature sensitivity may be reduced by a careful design of the temperature compensation circuitry, or by keeping a mass flow sensor in a controlled thermal zone. Pressure sensitivity may be addressed by additional calibration for the pressures.
Another strategy is to auto-zero mass flow sensors whenever possible and necessary. This strategy is implemented in some of the commercial instruments, such as Agilent 7890A Gas Chromatograph. Auto-zero is performed by shutting off the MFC proportional valve for a short period of time (e.g., 1.5-6 seconds at the end of each run) to create a zero (or near zero) flow condition. During the shut off, the mass flow sensor takes a measurement, and the newly acquired sensor signals are used to update the zero-offset values. Auto-zero is an effective way to correct for the long-term drifting or other changes (e.g., sudden changes). In addition, auto-zero is also a feasible approach to addressing the temperature and pressure variations.
However, the need to shut off the MFC proportional valve for auto-zero calibration means that auto-zero can only be safely carried out between active measuring or analytical processes, because flow control interruptions may produce detrimental effects. Furthermore, proportional valves in MFCs are usually not positive shut off valves. As a result, a certain amount of leak is always present, and any leak would introduce errors in the zero-offset. To ensure absolutely zero flow during an auto-zero process, additional positive shut off valves are required, either upstream or downstream or on both sides of the MFC. Such additional positive shut-off valves increase the overall system costs and complexity.
U.S. Pat. No. 5,542,286, issued to Wang et al., discloses a method of correcting flow and pressure sensor drifts in a gas chromatograph. In one of the embodiments described, when the GC is not being used, the input valve is shut, reducing the internal flow to zero. The indicated rate of flow is then measured using the flow sensor. In situations where the flow should not be interrupted, a three-way valve that can be used to direct flow away from the flow sensor during the calibration is described.
In the apparatus of the '286 patent, the flow controller is used to control total flow into the inlet. The low flow rates required for the chromatographic column and the septum purge flows are controlled by controlling the pressure to the column and the septum purge regulator. Because of this specific design and the apparatus uses the flow sensor to control total flow, changes in total flow from the flow controller may occur when the flow sensor is being bypassed. The '286 patent does not address ways to maintain the total flow at a constant value during calibration.
There are applications where it is important to control low flows of gases that cannot use pressure values to control the proportional valve during calibration. For applications that require accurate and precise control of low flow rates, there is a need to calibrate the sensor while continuing to adequately control the flow.